Sunday, March 01, 2009

Uncertain Conclusions

This post concerns serious business, so I'll start with some levity.
A salesman, a hardware engineer, and a computer programmer were in a company car traveling down a steep mountain road when the car's brakes gave out. Miraculously, the driver was able to bring the car to a stop by turning off the engine and using the emergency brake.

As they piled out of the vehicle to assess the damage the salesman declared, "We should call the company carpool and have them dispatch us a brand new company car."

The hardware engineer shook his head and said, "No, we should take this car apart, find out what went wrong, get new parts, and fix this car."

The programmer rolled his eyes and opined, "No, no, no. We should take this car back up the hill and see if it does it again."

There are limits to inductive reasoning, especially when one tries to draw conclusions based on a very small sample of events. It's easy to draw a conclusion from experience, but that conclusion could be invalid or incomplete, depending on your experience and, among other things, luck. Pilots often receive in-depth training in aircraft systems and that can lead them to believe they can troubleshoot or fix problems in flight, but don't forget an important part of aeronautical decision making: Have a solid respect for what you don't know and avoid making unnecessary and risky assumptions.

Consider the procedure for resetting a popped circuit breaker while flying an aircraft, a practice that is now virtually prohibited in commercial aviation but may still be widely practiced in general aviation aircraft. A related problem involves flying an aircraft with non-essential, inoperative equipment that hasn't been removed or deactivated. The NTSB has concluded that both of these issues likely led to the crash of the Cessna 310 I mentioned in my last post.

Like many things, the FAA's regulations don't make the process of dealing with inoperative equipment a simple matter. There are seven (count 'em!) steps you may need to follow to be legal.
  1. Does the aircraft in question have a minimum equipment list (MEL) and, if so, is the thing that's inoperative required on the MEL? If yes, the aircraft is grounded and corrective action is required. If no, then skip to step 6. If your aircraft doesn't have an MEL, then ...
  2. Does the thing that's inoperative appear in the aircraft's original equipment list as being required for certification? If yes, the aircraft is grounded and corrective action is required. Otherwise ...
  3. Does the thing that's inoperative appear in the aircraft's Kinds Of Operation Equipment List (if the aircraft has one) found in the Limitations Section of the AFM for the planned type of operation? If yes, the aircraft is grounded and corrective action is required. Otherwise ...
  4. Is the thing that's inoperative required by 14 CFR 91.205 for the planned type of operation? If yes, the aircraft is grounded and corrective action is required. Otherwise ...
  5. Is the thing that's inoperative required by an Airworthiness Directive? If yes, the plane is grounded and corrective action is required. Otherwise ...
  6. Do you, as PIC, believe that safety of flight will be adversely affected by the thing that is inoperative? If yes, the aircraft is grounded (see Sam's blog for a great description of a PIC's real life safety-of-flight decision). Otherwise ...
  7. Deactivate or remove the thing that is inoperative, placard it as inoperative, make a maintenance log entry describing what you did, and you can now go flying.
An important thing to consider is "When did the thing that is inoperative actually quit working?" Often you'll discover something is broken during your preflight inspection, taxi check, or during the before takeoff checks. If you discover something is broken while in flight, you should tell someone and make a record of the discrepancy after you complete your flight. This discrepancy report (US pilots often call them squawks) should alert the pilot who is scheduled to fly the aircraft next, provided that information is recorded somewhere and communicated to maintenance personnel and the next pilot(s).

The FAA tries to incorporate these maintenance concepts in the various Practical Test Standards under the Emergency Operations and Postflight Procedures areas of operation. In my experience, few GA pilots actually complete a postflight inspection after flying. Heck, with some pilots you're lucy if they remember to tie the plane down, install the gust lock, and so on. Many designated pilot examiners will thoroughly quiz a candidate about required equipment and the handling inoperative equipment, but a lot of pilots seem to forget these procedures as soon as the ink is dry on their temporary airman's certificate. Even the very experienced airline transport pilot involved in the Cessna 310 crash seems to have overlooked the proper procedure for handling inoperative equipment.

A maintenance discrepancy log sheet recovered at the accident site, annotated during the previous day’s flight, described the weather radar display as going “blank” during cruise flight, accompanied by a “smell of electrical components burning. Turned off unit - pulled radar [circuit breaker] - smell went away.” No corrective action was annotated next to the discrepancy write-up, and no evidence was found to indicate that corrective action was taken prior to the mishap flight.

Post accident interviews with company personnel indicated that during a phone conversation the day prior to the accident, the ATP was made aware of the weather radar discrepancy item. A company aircraft mechanic confirmed that the ATP stated that he “didn’t care about the radar” during a telephone conversation on the morning of the accident.

None of the company personnel interviewed remembered seeing either member of the accident flight crew reviewing the airplane discrepancy log prior to departure, but one mechanic did recall that the ATP performed the preflight inspection of the airplane.

The pilots decided to take the plane without thoroughly investigating the squawk about the radar and it appears the circuit breaker that had been pulled was not "collared" by maintenance to prevent it being reset. Resetting the circuit breaker may have caused the Cessna 310's in-flight fire. Here's how.

Older style circuit breakers use a bimetallic, thermal design that responds to the heat generated when a specified electrical load is exceeded. A metal collar holds the circuit closed against the force of a spring contained inside the circuit breaker. Should the collar begin to heat up, it will expand, allowing the spring to open the circuit. This design generally works, but it has two problems.

Old circuit breakers may no longer perform as designed, especially if they have accumulated a coating of dirt, dust, or some sort of spilled contaminant like coffee or soft drinks.

Even if circuit breakers aren't contaminated, an arcing event between wires whose insulation has chaffed away may not generate enough heat at the breaker to cause it to open immediately. Significant arcing damage may occur and significant smoke can be generated before a conventional circuit breaker will trip. Arc-Fault Circuit Breakers are now available which can detect arcing events and immediately open the circuit before significant damage occurs, but you can bet that 70's, 80's or 90's vintage aircraft are probably not equipped with such devices.

According to AC 120-80, most hidden fires are the result of arcing between bundled wires. Uncontaminated wiring insulation is fire resistant, but an arc can easily provide a source of ignition for surrounding insulation materials or even wiring insulation if it has been contaminated with dust/dirt or stray corrosion inhibitors and lubricants. Once a circuit breaker trips, you may very well be confronted by an electrical system in an indeterminate state: The AFM's wiring diagram may no longer apply to your aircraft.

The problem is made worse by the fact that some aircraft designs put circuit breakers in very difficult-to-reach places. The Caravan has numerous rows of circuit breakers located on a panel adjacent to the pilot's left leg, leading some operators put color-coded plastic collars on important circuit breakers to help pilots figure out which circuit has opened. The Cirrus SR20 and SR22 have circuit breakers that are located near the pilots right foot in a location where parallax makes it virtually impossible to accurately read the labels while flying the aircraft. Piper Comanche and Twin Comanche aircraft have circuit breakers behind a trap door on the floor, another illustration of a fundamentally bad design decision.

Did the pilots of that Cessna 310 reset the circuit breaker for the radar equipment that had previously been pulled (but not collared) by the previous pilot? We may never know.
At 08:32:50, shortly after reaching a cruising altitude of 6,000 feet, the flight crew declared an emergency to the Orlando International approach air traffic controller (ATC). The crew advised that there was "smoke in the cockpit," and announced their intention to land at SFB. ATC cleared the airplane to fly directly to SFB and descend to 2,000 feet. Radar data indicated that the accident airplane turned toward SFB and commenced its descent. ATC then cleared the accident crewmembers to “to land any runway." The last radio transmission from the airplane occurred at 08:33:15. It was terminated in mid-sentence and appeared to include the phrase “shutoff all radios, elec.” The last radar return from the accident airplane was at 08:34:50, about 1/2-mile
east of the accident site.

GA pilots can learn a lot from this accident by following a procedure for fires and tripped circuit breakers that the airlines generally follow, as outlined in AC 120-80. First, be aware of the indications of hidden fires:
  • Odors of smoke or of hot electrical equipment
  • Un-commanded operation of an aircraft system may indicate a hidden fire
  • Circuit breakers tripping
  • Hot spots on the floor, or behind sidewalls and access panels
  • Smoke or visual signs of flames
Should a fire develop, be prepared to follow these steps:
  • Fight the fire immediately, be aggressive
  • Try to find the base or source of the fire
  • Do not reset circuit breakers unless the equipment is required for safe flight
  • Plan for an immediate descent and landing at the nearest suitable airport
  • Use personal breathing equipment (supplemental oxygen) if available
Be wary if a circuit breaker trips and try to apply all that systems knowledge you learned once you are safely on the ground. Knowing the signs of in-flight fires, how to properly handle inoperative equipment, leaving tripped circuit breakers alone, and being familiar with using fire extinguishers may save your life some day.


Anonymous said...

I was watching one of the instructional videos by John and Martha King and they said that one fire-fighting technique in a light plane is to dive the plane to try to put out the fire. Is that something that you teach your students or have you heard of that technique?

John said...

Most manufacturer's aircraft flight manuals give a procedure for descending at a fairly high rate of speed as a way of extinguishing an engine fire and this is a technique that I teach. This procedure probably won't help if the source of the fire is inside the aircraft cabin.